The present invention relates to communication signal processing. More specifically, the present invention relates to the regeneration of the in-phase and quadrature signal components received by communication receivers for digitally modulated signals.
In many communication, radar or instrumentation applications, a receiver generates two orthogonal signal components of the received signal to ease recovery of information contained within the received signal. Generally the two orthogonal baseband signals are generated from an incoming signal having a centre frequency (CF) by mixing the incoming signal with a reference signal tuned to approximately a same CF. The in-phase (I) signal results from mixing the incoming signal with the reference signal tuned to approximately a same CF and the quadrature (Q) signal results from mixing the incoming signal with a version of the reference signal phase shifted by 90xc2x0. After mixing the signals, a filter is used to remove undesired products of the mixing. Amplification is commonly used to adjust the I and Q signals to suitable levels for further processing.
In implementing circuitry to perform the mixing, filtering and amplification for each of the I and the Q signal paths, it is difficult to obtain a perfect match between the paths for phase, gain and DC offsets. Consequently distortions are present between the I and Q signals resulting in distortion/corruption of information carried by the incoming signal. For example, phase imbalances between, gain imbalances between and DC offsets on the I and Q signals may affect data derived from these signals resulting in errors in the derived data. In order to minimise the effects of the circuit imperfections on the integrity of the recovered information from the incoming signal, a method is required to compensate for circuit imbalances between the paths and the DC offsets.
In the past, these problems were avoided by using stringent specifications on components, and by tuning the paths to minimise imbalances therebetween and DC offsets. This is therefore more prone to degradation, more expensive and so forth.
An approach to I and Q signal regeneration according to the prior art uses a six-port junction based direct receiver (hereafter xe2x80x9c6-port receiverxe2x80x9d). Using such a receiver and a two-tone calibration technique initially developed for reflectometry applications, an incoming signal is split into three separate signals and a fourth output port provides a signal proportional to the reference signal. These are then used for generation of an I component signal and a Q component signal. The function of such a receiver requires that an incoming signal is split into three versions each phase shifted from the other two versions by 120 degrees. This technique has specific requirements on the incoming signal to perform its calibration and is computationally intensive for real time applications.
The primary object of the invention is to provide a method for regenerating IandQ signals from an incoming signal in the presence of gain imbalances, phase imbalances and DC offsets in regeneration circuitry. The method is applicable to signals either at baseband or at another centre frequency.
Another object of the present invention is to compensate in the receiver for signal distortion due to gain and phase imbalances in the generation circuit at a transmitter.
In accordance with the invention, there is provided a method of in-phase signal regeneration and quadrature signal regeneration, the method compensating for phase imbalances, gain imbalances and DC offsets, the method comprising the steps of:
receiving a signal;
generating three digitised phase shifted versions of the received signal, the versions phase shifted one from the others by an amount other than 0 and 180 degrees, the digitised phase shifted versions each comprising a plurality of samples;
determining from the samples a mean value for each signal, the mean values for compensating for DC offsets;
determining from the samples a plurality of coefficients for use in mapping the three digitised phase shifted versions of the received signal into dimensions;
projecting data derived from each of the three digitised phase shifted versions onto a two-dimensional signal subspace wherein samples from each digitised phase shifted version are multiplied by a determined coefficient and combined in a linear fashion.
In accordance with another embodiment of the invention, there is provided a method of in-phase signal regeneration and quadrature signal regeneration, the method compensating for phase imbalances, gain imbalances and DC offsets, the method comprising the steps of:
receiving a signal;
generating three digitised phase shifted versions of the received signal, the versions substantially similar to digitised versions obtained by sampling three identical signals phase shifted one from the others by an amount other than 0 and 180 degrees; and,
projecting data derived from each of the three digitised phase shifted versions onto a two-dimensional signal subspace wherein data derived from each digitised phase shifted versions affects a resulting signal within each of the two dimensions of the subspace.
In accordance with another embodiment of the invention, there is provided a method of in-phase signal regeneration and quadrature signal regeneration, the method compensating for phase imbalances, gain imbalances and DC offsets, the method comprising the steps of:
receiving a signal;
generating a plurality of digitised phase shifted versions of the received signal, the versions phase shifted one from the others by an amount other than 0 and 180 degrees;
generating a correlation matrix, the correlation matrix dependent upon the plurality of digitised phase shifted versions;
decomposing the correlation matrix to produce in-phase and quadrature regeneration coefficients; and
applying the coefficients to the digitised phase shifted versions of the signal to extract in-phase and quadrature components of the received signal therefrom.
In accordance with yet another embodiment of the invention, there is provided a method of in-phase and quadrature signal regeneration for use with one of a quadrature receiver and a quadrature transmitter, the method compensating for phase imbalances, gain imbalances and DC offsets in the one of the quadrature receiver and a quadrature transmitter, the method comprising the steps of:
receiving a signal;
generating a plurality of digitised phase shifted versions of the received signal;
processing the digital versions to determine linear regeneration coefficients; and,
applying the determined linear combination coefficients to the digitised phase shifted versions to extract in-phase and quadrature signal components from the received signal.
In an embodiment, processing the digital versions to determine linear regeneration coefficients comprises the steps of:
determining a correlation matrix of the digitised phase shifted versions over a set of samples of a predetermined size;
determining a mean value of each digitised phase shifted version over the set of samples;
determining eigenvalues of the determined correlation matrix;
ranking the determined eigenvalues and selecting two of the eigenvalues;
determining eigenvector components associated with the two selected eigenvalues;
mapping the eigenvector components to regeneration coefficients;
determining the DC offset compensation coefficients using the regeneration coefficients; and,
providing the coefficients as the linear combination coefficients.
In an embodiment, applying the determined linear combination coefficients comprises the following steps:
receiving the regeneration coefficients;
multiplying samples from the digitised phase shifted versions with associated regeneration coefficients to produce results and summing the results to produce an estimate of samples of the in-phase and quadrature signal components; and
summing the DC compensation coefficients with the estimate of samples of the in-phase and quadrature signal components to produce samples of the in-phase and quadrature signal components.
In accordance with another aspect of the invention, there is provided an in-phase and quadrature signal regeneration circuit for use with one of a quadrature receiver and a quadrature transmitter, the circuit compensating for phase imbalances, gain imbalances and DC offsets in the one of the quadrature receiver and the quadrature transmitter, the circuit comprising:
an analog to digital converter for generating a plurality of digitised phase shifted versions of a signal received by the quadrature receiver;
a processor for processing the digital versions to determine linear regeneration coefficients and for applying the determined linear combination coefficients to the digitised phase shifted versions to extract in-phase and quadrature signal components from the received signal.